arXiv subjects

Donald E. Neville

Publications and source records attributed to Donald E. Neville.

13 recordsLinked to original sources

arXiv2014

Plane wave holonomies in loop quantum gravity I: symmetries and gauges

This is the first of two papers which study the behavior of the SU(2) holonomies of loop quantum gravity (LQG), when they are acted upon by a unidirectional, plane gravity wave. Initially, the LQG flux-holonomy variables are treated as classical, commuting functions rather than quantized operators, in a limit where variation from vertex to vertex are small and fields are weak. Despite the weakness of the fields, the field equations are not linear. Also, the theory can be quantized, and the expectation values of the quantum operators behave like their classical analogs. Exact LQG theories may be either local or non-local. The present paper argues that a wide class of non-local theories share non-local features which survive to the semiclassical limit, and these non-local features are included in the classical limit studied here. An appendix computes the surface term required when the propagation direction is the real line rather than $\mathrm{S}_1$. Paper II introduces coherent states, constructs a damped sine wave solution to the constraints, and solves for the behavior of the holonomies in the presence of the wave.

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arXiv2014

Plane wave holonomies in loop quantum gravity II: sine wave solution

This paper constructs an approximate sinusoidal wave packet solution to the equations of loop quantum gravity (LQG). There is an SU(2) holonomy on each edge of the LQG simplex, and the goal is to study the behavior of these holonomies under the influence of a passing gravitational wave. The equations are solved in a small sine approximation: holonomies are expanded in powers of sines, and terms beyond $\sin^2$ are dropped; also, fields vary slowly from vertex to vertex. The wave is unidirectional and linearly polarized. The Hilbert space is spanned by a set of coherent states tailored to the symmetry of the plane wave case. Fixing the spatial diffeomorphisms is equivalent to fixing the spatial interval between vertices of the loop quantum gravity lattice. This spacing can be chosen such that the eigenvalues of the triad operators are large, as required in the small sine limit, even though the holonomies are not large. Appendices compute the energy of the wave, estimate the lifetime of the coherent state packet, discuss coarse-graining, and determine the behavior of the spinors used in the U(N) SHO realization of LQG.

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arXiv2013

Spin network coherent states for planar gravitational waves. I

This paper constructs coherent states for spin networks with planar symmetry. After gauge-fixing, the full SU(2) symmetry is broken to U(1), but one cannot simply use the U(1) limit of SU(2) coherent states, because the planar states exhibit an unexpected O(3) symmetry arising from the closed loop character of the transverse directions. The coherent states constructed in this paper obey this symmetry. They are superpositions of holonomies which obey the residual U(1) symmetry only on average; some holonomies in the superposition violate the symmetry, although the U(1) quantum numbers of these holonomies are peaked at values which obey the symmetry. Operators acting on coherent states give back a c-number times the original state, plus small correction states, which make the coherent state an approximate, rather than exact eigenstate of the operator. In a follow-on paper, these small correction states are used to calculate small corrections to the volume operator.

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arXiv2013

Planar spin network coherent states II. Small corrections

This paper is the second of two which construct coherent states for spin networks with planar symmetry. Paper 1 constructs set of coherent states peaked at specific values of holonomy and triad. These operators acting on the coherent state give back the coherent state plus small correction (SC) states. The present paper proves that these SC states form a complete subset of the overcomplete set of coherent states. The subset is used to construct a perturbation expansion of the inverse of the volume operator. Appendices calculate the standard deviations of the angles occurring in the holonomies, demonstrate that standard deviations are given by matrix elements of the SC states, and estimate the rate of spreading of a coherent state wave packet.

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arXiv2008

On the Classical Limit of Spin Network Gravity: Two Conjectures

Estimates are given of the time scales which govern spreading of a coherent state wave packet. The estimates, based on dimensional analysis, suggest that spreading should be small for coherent states with average angular momentum of order 100 or larger. It is conjectured that in the classical limit, terms in the Hamiltonian which add a new vertex will be suppressed, compared to terms which modify the existing spin network without changing the number of vertices.

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arXiv2006

The volume operator for singly polarized gravity waves with planar or cylindrical symmetry

A previous paper constructed a kinematic basis for spin networks with planar or cylindrical symmetry and arbitrary polarization. This paper imposes a constraint which limits the gravitational wave to a single polarization. The spectrum of the constraint contains a physically reasonable number of zero eigenvalues, and the zero eigenvectors can be constructed explicitly. Commutation of the constraint with the Hamiltonian is expected to lead to a further constraint. This new constraint is not investigated in this paper, but I argue it will be non-local, relating states at two or more neighboring vertices.

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arXiv2006

The volume operator for spin networks with planar or cylindrical symmetry

This paper constructs a kinematic basis for spin networks with planar or cylindrical symmetry, by exploiting the fact that the basis elements are representations of an O(3) subgroup of O(4). The action of the volume operator on this basis gives a difference equation for the eigenvalues and eigenvectors of the volume operator. For basis elements of low spin, the difference equation can be solved readily on a computer. For higher spins, I solve for the eigenvalues using a WKBJ method. This paper considers only the case where the gravitational wave can have both polarizations. The single polarization case is considered in a spearate paper.

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arXiv1998

Long range correlations in quantum gravity

Smolin has pointed out that the spin network formulation of quantum gravity will not necessarily possess the long range correlations needed for a proper classical limit; typically, the action of the scalar constraint is too local. Thiemann's length operator is used to argue for a further restriction on the action of the scalar constraint: it should not introduce new edges of color unity into a spin network, but should rather change preexisting edges by $\pm$ one unit of color. Smolin has proposed a specific ansatz for a correlated scalar constraint. This ansatz does not introduce color unity edges, but the [scalar, scalar] commutator is shown to be anomalous. In general, it will be hard to avoid anomalies, once correlation is introduced into the constraint; but it is argued that the scalar constraint may not need to be anomaly-free when acting on the kinematic basis.

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arXiv1997

Energy and directional signatures for plane quantized gravity waves

Solutions are constructed to the quantum constraints for planar gravity (fields dependent on z and t only) in the Ashtekar complex connection formalism. A number of operators are constructed and applied to the solutions. These include the familiar ADM energy and area operators, as well as new operators sensitive to directionality (z+ct vs. z-ct dependence). The directionality operators are quantum analogs of the classical constraints proposed for unidirectional plane waves by Bondi, Pirani, and Robinson (BPR). It is argued that the quantum BPR constraints will predict unidirectionality reliably only for solutions which are semiclassical in a certain sense. The ADM energy and area operators are likely to have imaginary eigenvalues, unless one either shifts to a real connection, or allows the connection to occur other than in a holonomy. In classical theory, the area can evolve to zero. A quantum mechanical mechanism is proposed which would prevent this collapse.

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arXiv1997

Total intrinsic spin for plane gravity waves

A quantity which measures total intrinsic spin along the z axis is constructed for planar gravity (fields dependent on z and t only), in both the Ashtekar complex connection formalism and in geometrodynamics. The total spin is conserved but (surprisingly) is not a surface term. This constant of the motion coincides with one of four observables previously discovered by Husain and Smolin. Two more of those observables can be interpreted physically as raising and lowering operators for total spin.

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arXiv1996

Closed-Flux Solutions to the Constraints for Plane Gravity Waves

The metric for plane gravitational waves is quantized within the Hamiltonian framework, using a Dirac constraint quantization and the self-dual field variables proposed by Ashtekar. The z axis (direction of travel of the waves) is taken to be the entire real line rather than the torus (manifold coordinatized by (z,t) is RxR rather than $S_1$ x R). Solutions to the constraints proposed in a previous paper involve open-ended flux lines running along the entire z axis, rather than closed loops of flux; consequently, these solutions are annihilated by the Gauss constraint at interior points of the z axis, but not at the two boundary points. The solutions studied in the present paper are based on closed flux loops and satisfy the Gauss constraint for all z.

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arXiv1995

Open-Flux Solutions to the Constraints for Plane Gravity Waves

The metric for plane gravitational waves is quantized using the Ashtekar field variables. The z axis (direction of travel of the waves) is taken to be the entire real line. Solutions to the constraints are proposed; they involve open-ended flux lines running along the entire z axis. These solutions are annihilated by the constraints except at the two boundary points, where the Gauss constraint does not annihilate the solutions. This result is in sharp contrast to the situation in the general, 3+1 dimensional case without planar symmetry, where the Gauss constraints do not contribute at boundaries because the Lagrange multipliers for the Gauss constraints vanish there. The constraints annihilate the solutions if classical matter is included (so that flux lines are terminated on the matter). The SU(2) holonomy matrices used in the solutions are (2j+1) dimensional, where j may be any spin, not necessarily j = 1/2. In this respect the solutions resemble the spin network states recently constructed by Rovelli and Smolin in loop space.

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arXiv1995

Closure of Constraints for Plane Gravity Waves

The metric for gravitational plane waves has very high symmetry (two spacelike commuting Killing vectors). For this high symmetry, a simple renormalization of the lapse function is found which allows the constraint algebra for canonical quantum gravity to close; also, the vector constraint has the correct form to generate spatial diffeomorphisms. A measure is constructed which respects the reality conditions, but does not yet respect the invariances of the theory.

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